A semiconductor integrated circuit in which both of a digital circuit and an analog circuit are mounted on an identical wafer is known. A digital circuit is expected to achieve high speed, high density, and low power consumption. In order to achieve these goals, miniaturization of a MOS transistor has been attempted in general. Reduction in the gate length of a MOS transistor for the purpose of miniaturization causes threshold voltage to decrease due to short-channel effect and consumed power to rapidly increase. In order to prevent the rapid increase in consumed power, a MOS transistor is, by performing halo implantation, formed so as to increase impurity concentration around an end of the source region and an end of the drain region to a high level. As a result, in a MOS transistor on which halo implantation is performed, reverse short-channel effect occurs in which threshold voltage increases as gate length is shortened.
On the other hand, in an analog circuit, 1/f noise of MOS transistors often influence product performance substantially. The halo implantation, which is regularly performed on a digital circuit, causes 1/f noise to deteriorate. In addition, 1/f noise decreases as device area increases. In a circuit in which the influence of noise cannot be ignored, channel length is intentionally set long. Therefore, in an analog circuit, leakage current is unlikely to become a problem compared with a digital circuit. For this reason, in an analog circuit, the dose amount of impurity ions in halo implantation is sometimes reduced and in other cases halo implantation itself is not performed.